Method of applying metal coatings



Patented June 13, 1933 UNITED STATES PATENT OFFICE TADEUSZ LIBAN, OF GRACOW, POLAND METHOD OF APPLYING METAL COATINGS No Drawing. Application filed July 27, 1926, Serial No. 125,321, and in Germany August 18, 1925.

My invention relates to a new method of applying metallic coatings to ironor metal-articles and is particularly applicable in coating iron articles with zinc.

In order to obtain-a perfect coating it is absolutely necessary to previously clean the surfaces of the articles to be coated from the oxides and to keep them free from oxides during the whole process. The surfaces are therefore cleaned in the first place by acids,

lyes or by mechanical means. salts and impurities are then washed off in the water-bath.

Notwithstanding this preparatory treatment it is not always possible to quite remove the oxides. Besides new oxides are formed while the articles are washed in the Water-bath.

Again it is a very grave point that the coating-metal itself especially the zinc contains oxides and is likely to oxidise while in -a molten state. These oxides too hinder a perfect coating.

My invention does away with these difliculties. To this end I use with great suc cess fluorine-compounds as a means for dissolving the oxides and promoting the coating process. The use of the fluorine-compounds may take place in different stages of the process in a different way.

After passing the acid bath and the water bath the articles used to be dipped into a strong solution of zinc-chloride or'into a solution of a like character. They are then dried in a special kiln and finally introduced into the metal-bath. According to my invention fluorine-compounds are added to the said solution which by this means becomes much more effective in destroying the ox- 49 ides. Moreover this solution adheres now better to the surfaces of the articles and prevents them from becoming again oxidised.

In the same way solutions of fluorine-compounds only may be used.

Instead of dipping the articles into the protective solution their surfaces may be powdered or coated with a protective layer of salts of fluorine or of a mixture of these salts with zinc-chloride, ammonium-chlo- 50 ride or the like ,as well as with resins, fats or The adherent the like. The articles are then being metal-coated.

The dipping of the articles into a molten bath of protective salts is very effective.

dried before *This bath consists according to my invention of fluorine-compounds or of mixtures of fluorine-compounds with salts of Zinc-chloride or the like andin some instances also with resins, fats or the like.

Owing to the high temperature the saltbath cleans intensively andperfectly the surfaces of'the articles treated from the oxides and the protective layer formed on these surfaces prevents in a high degree a new oxida tion. -At the same time the articles dipped into the hot bath are dried and can be brought directly into the metal-bath without anyintermediate treatment.

It is very important to" remove the dross and the other oxides from the surface of the metal-bath during the coating-process. This maybe attained without difficulty by means of flourine-compounds or by a mixture of these compounds with-salts of zinc-chloride,

ammonium-chloride or the like or with resins, fats, sand or the like. To this end the metalbath is covered with a layer of the said salts or of a mixture of the salts. These means for cleaning from oxides may be removed after some time. then metalically clean.

In applying zinc-coatings to iron articles a constant protective layer is used to be em ployed which consists of molten salts of zincchlorides, and through which the articlcs to 35 be treated are passed into the zinc-bathl This protective layer is produced during the application of the zinc-coatings by the decom position of ammonium-chloride upon the surface of the molten metal at a temperature of about 450 C.

The following better illustration The salts of zinc-chlorides thus formed melt at thetemperature of the zinc-bath to a liquid salt-mass floating on the surface of the equation may serve for The surface of the. bath is 80 v in a wet state from the treatment with acid are completely dried and preheated on being passed through the molten, fusible mass of zinc-chlorides. Moreover by these means the articles to be coated can be dipped into the molten metal-bath without any danger.

As far as crude zinc without a considerable addition of aluminium is used for the zinccoatings the production and the use of a protective layer consisting exclusively of salts of zinc-chlorides are practicable without difliculty. But if the zinc bath contains even a few tenths of percents of metallic aluminium, the use of a protective layer consisting solely of salts of zinc-chlorides is quite impossible, for after a relatively short'time the said protective layer will be converted into a solid plastic mass not suitable for the application of metal coatings. The mass of salt growing solid generates at the same time much vapors of ammonium-chloride which make it impossible to abide in the working-rooms.

But it is very desirable to add aluminium to the zinc-bath, as the aluminium; reduces the zinc-oxides of the bath to metallic zinc. The more the zinc-bath contains oxides the more it is diflicult to make it fusible. The quantity of the aluminium to be added depends.

therefore upon the quantity of oxides contained in the zinc-bath.

Moreover the zinc-bath is constantly enriched on its surfaces by oxides which can be removed only by a constant addition of aluminium. I prefer to add the aluminium in the form of a zinc-aluminium alloy containing-but a few percents of aluminium. An alloy of such a proportion does not sensibly change the amount of metallic aluminium contained in zinc-bath which is preferably O.30.5%. The solid block of this alloy dissolves relatively quick, and has the remarkable advantage of being specifically heavier than the molten zinc bath, so that it drops on the bottom of the bath and its solution takes place underneath the articles dipped into the bath.

By these means the zinc-bath remains constantly clean, fusible and by the reduction of the zinc-oxides only very small quantities of aluminium-oxides are formed.

The aluminium used for zinc-coating in the from of zinc-aluminium alloys cleans the zincbath by its deoxidizing effect. The zinc-bath becomes more fusible and gives remarkably adherent metal coatings which may be bent and folded. Moreover the use of aluminium in applying zinc-coatings much zinc is saved.

due to the fusibility of the metal-bath emme from which the immersed articles take less zinc.

But since the protective layer of salts of zinc-chlorides is'of no use when aluminium is added to the zinc-bath, it was necessary to have recourse to troublesome and expensive expedients in order to attain the advantages of this method of applyin zinc-coatings. The course was adopted to dry the articles to be zinc-coated and to perform the coating operation without any protective layer. After pickling the articles are dipped into a strong lye of zinc-chloride and then dried .in special kilns. When dry they are covered with a thin layer of zinc-chloride and are since those portions of the iron which have not been in contact with the lye of zinc-chloride, or from which the dried layer of zinc-chlorid has become chafed off, will be ceated with the metal but partially.

In addition it is to be pointed out that by the mere immersion of the articles to be coated into a lye-of zinc-chloride having a temperature of 16 C. the cleaning-from oxides cannot be carried out as perfectly as at a temperature of 450 C. by a fusible well adhering molten mass of salt. 7

The present invention does entirely away with all those difficulties and insufliciencies. To this end according to the present invention during the process of the application of zinc-coatings a protective layer is produced which consists of a mixture of salts of zinc-chlorides and of fluorine-compounds.

A salt-mixture of ammonium-zinc-chlorides and fluorine-compounds melts upon the surface of the bath of zinc having a large percentage of aluminium to a readily fusible mass of salt, which generates no vapors, which does not decompose, which exerts an oxide cleanin efiect, and brings about the application 0 zinc-coatings in an excellent and easy manner. 7 1

The previous drying of the articles to be treated is quite superfluous with the process I of the invention. The new protective layer absorbs all moisture from the goods to be treated so that the said goods enter the molten zinc-bath containing aluminium in a -perfectly dry state, preheated and metallically pure.

The articles are drawn into the metalforms Very objectionable vapors of aluminium-chloride. But these vapors are apt to combine with alkali and to form a fusible salt of alkali-aluminium-chloride according to the following equation:

KF NH Cl KCl NILF 2KF ZnCl 2KC1+ ZnF KCl A1 C1 KCLAlCl This suppression of obnoxious vapors is further favoured by the formation of alkalialuminium-fiuorides and other double-fluorides of aluminium, by the solution of the aluminiuin-oxide in the double-fluorides, the formation of zinc-fluorides, ammonium fluorides and the double-compounds of the latter,

as shown by the following equation:

The proportion of the several ingredients of the protective layer containing fluorine depends on the amount of aluminium con-- tained in the zinc-bath. An average proportion of O.56% of fluorideof potassium, 10- 20% of ammonium-chloride and 74.-89.5% of zinc-chloride will do.

The protective layer consisting of fluorinezinc-salts may also be used in applying coatings with crude zinc without any addition of aluminium. It has as against the protective layer of ammoniuni-chloride the great advantage'that it withdraws less zinc from the metal-bath. As a consequence its use is connectedwith smaller losses of zinc as the use of the known protective layers.

Moreover the cleaning effect of the protective layer containing salts of fluorine and zinc is more intensive and gives considerably better and more uniform coatings of zinc.

the said process a dross, containing at an average about percent of zinc, which was withdrawn from the bath of zinc.

The figures mentioned were obtained under equal conditions and methods of working.

The use of the fluoric protective layer has further proved practicable and better in aplying zinc-coatings according to the leadoath process than the protective layer of ammonium chloride and zinc-chloride. According to the lead-bath process only at the emerging point of the articles treated a floating film of zinc is present. Owing to the very strong oxide dissolving effect of the fluoric layer, the articles to be treated enter the metal-bath in a metallically pure condition. Furthermore the fluoric layer forms considerably less chloride of plumbum, than the layer of ammonium chloride and zinc-chloride. The articles coated according to the lead-bath process witha protectivefluoric layer show a-smooth, well-adhering, homogeneous coating of zinc. On Sherardizing a considerable improvement of the process is attained, in that in lieu of pure dust of zinc a mixture of dust of zinc and fluorine compounds, as for instance fluorid of calcium, .is used.

This mixture may consist of 90 percent of fluoride of calcium and 10 percent of zincpowder. It will yield a more homogeneous, denser, lighter and more adhering coating of .zinc free from pores than the pure zinc- ;powder without this admixture of fiuorlnecompounds.

The addition of fluoride of calcium or any other fluorine compounds keeps the'zinc-powder in a loose condition, prevents spontaneous combustion of the zine-powder in the air, and allows also to keep during the process a proper temperature, which lies 150 degrees to 200 degrees Centigrade below the fusing point of the zinc.

The mixture of fluorine and zinc-powder renders the Sherardizing operation cheap since with the proportion of mixture mentioned the loss in material and zinc is very small.

1 he machines for carrying out the process of metal-coating are provided with rotating rollers which on one side introduce the iron sheets or the other articles into the bath and remove them on the other side after the coating has taken place. in the metal bath and partly in order to be cleaned from oxides in a bath of salts, fats or resins. It is very important to keep the rollers thoroughly clean as otherwise the sheetstreatcd become stained while leaving the metal-hath. According to the invention compounds of fluorine are added to the said cleaning salts which now become more eifective and which in some instances become effective in which they have hitherto not been effective altogether.

In proposing in this specification the use These rollers dip partly 4 v amazes of fluorine-compounds, I did not for the sake of succinctness always mention their double salts, their complex compounds and mixtures with other salts. But I wish it to he understood that these double-salts, complex compounds and mixtures are within the scope of my invention as they contain fluorinecompounds.

'What I claim is:

1. The process of applying zinc coating to iron articles which consists in forming a bath of molten zinc providing said bath with a molten supernatant layer of zinc chloride, ammonium chloride and a fluorine compound and then passing the articles through said layer and bath successively.

2. The process of applying zinc coating to iron articles which, consists in forming'a 2w bath of molten zinc, adding'aluminum to I the zinc bath; forming, on the said bath, a

molten layer of zinc chloride, ammonium chloride and a fluorine compound; and finally passing the iron articles through said layer and bath successively.

3. The process of applying zinc coating to iron articles which consists in forming a bath of molten zinc; immersing in said zinc bath, a solid zinc aluminum alloy, which is specifically heavier than the zinc of the bath;

forming a molten layer of zinc chloride, am-

monium chloride and a fluorine compound on" said bath, and finally passing the articles through the said layer and bath successively. 4. The process of applying zinc coating to iron articles which consists in forming a bath of molten lead; forming a molten layer of zinc floating on the molten lead; forming on the molten lead a protective layer sepaeo rated from the molten layer of zinc and comprising a mixture of zinc chloride, ammonium chloride and a fluorine compound; passing the articles through the said protective layer into the bath; and extracting the articles from said bath through said zinc layer. 5. The process of. applying zinc coating to iron articles which consists in forming a bath of molten lead; forming a molten layer of a zinc-aluminum alloy floating on the W molten lead; forming on the molten lead a protective layer separated from the molten layer of zinc and comprising a mixture of zinc chloride, ammonium chloride and a fluorine compound; passing the articles through the said protective layer into the bath; and extracting the articles from said bath through said layer.

In testimony whereof I aflix my signature.

m TADEUSZ LIBAN. 

